1. Field of the Invention
The present invention concerns a device for monitoring a turret of a cryotank filled with liquid gas, with superconducting coils of a cryomagnet being located in the cryotank. Such cryomagnets are in particular used in magnetic resonance (MRT) apparatuses in medical imaging.
2. Description of the Prior Art
In recent years MRT apparatuses have increasingly gained importance in medical imaging. In order to produce images with such apparatuses, different magnetic fields that are tuned to one another as precisely as possible in terms of their spatial and temporal characteristics are typically combined with one another. One of thee magnetic fields is a static magnetic field with a high field strength of typically 1.5 to 3 Tesla and even more. In order to generate such high field strengths, usually a cryomagnet is employed that has conductors formed of a superconducting material, for example a niobium-titanium alloy. The cooling of the conductor wires and the maintenance of the superconducting state ensue with liquid helium. The conductors are thereby located in a cryotank. The cryotank typically has a tube or filling through which the liquid helium can be filled into the cryotank.
An infrequent but serious event in a cryomagnet is a “quench.” A “quench” is the change of the superconducting state of the conductors to the normally conductive state, which can have different causes. The severe heating of the conductor wire that occurs in a quench leads to a rapid vaporization of the liquid helium. In order to avoid damage to the apparatus, the suddenly arising large volumes of helium gas must be quickly directed out from the cryotank so that the cryomagnet does not explode. The helium gas thereby escaping can also harm people (danger of asphyxiation). In cryomagnets, a quench tube is therefore typically connected with the cryotank, the quench tube typically achieving a connection of the cryotank with the exterior of the turret. The large volumes of helium gas arising given a quench are consequently conducted via the turret and via the quench tube from the inside of the cryotank to the outside. The diameter of the quench tube is typically 10 to 40 cm. The diameter of the turret can be smaller since given a quench helium gas is still very cold and only expands due to the heating in the course of the quench tube. In order to ensure the evacuation of the gas quantity, the turret and the quench tube may not be blocked.
A possible and particularly dangerous blockage is icing in the turret of the magnet. Such icing can occur, for example, due to leaks or due to operating errors in the refilling with liquid helium. The ice is composed of frozen air. An iced-over turret must be made passable again by removing the ice, for example by careful blowing of warm helium gas onto the ice. Otherwise the magnet could explode in a quench, which would damage the apparatus and would endanger people who remain in the environment of the magnet.
Conventionally a turret has been regularly checked by manual appraisal, for example in the framework of annual maintenance work. For this purpose, an opening in the turret is temporarily sealed with Plexiglas® and this is looked through, for example. Depending on the maintenance interval it can occur that the turret undetectably ices in the interval. If a quench then occurs, a risk to the apparatus and people exists.